The present invention relates to an electrophotographic method and also to an electrophotographic apparatus. More particularly, the invention is concerned with a method of and apparatus for conducting non-contact development of an electrostatic latent image by means of a toner.
In known non-contact type developing method as disclosed in Japanese Patent Unexamined Publication Nos. 60-242469 and 59-91453 employ a developing sleeve on which a developer layer is formed from a toner or a mixture of a toner and a carrier, and a photosensitive member which is disposed leaving a slight gap between the developer layer and the photosensitive member. In operation, a suitable level of bias voltage is developed across the gap so as to enable the toner to fly onto the photosensitive member, whereby an electrostatic latent image on the photosensitive member is developed.
The non-contact type developing method can broadly be sorted into two types: namely, (a) a mono-component layer type method in which the thin layer on the developer carrier is composed of a toner containing no carrier, and (b) a bi-component layer type method in which the thin layer is composed of a mixture of a toner and a carrier. The application of toner onto the developing roller is conducted by, for example, (a) a magnetic brush type method in which the toner is applied by a magnetic brush roll or by (b) an elastic blade which is pressed onto the developer carrier so as to apply the toner.
These known methods, however, suffer from the following disadvantages.
Namely, no specific consideration is given as to an effective pattern of distribution of the thickness of the developer layer on the developing sleeve. The size of the gap between the photosensitive member and the developing sleeve is determined by the spatial arrangement of the photosensitive member and the developing sleeve on the basis of the radii of the developing sleeve and the photosensitive member. Therefore, when the sleeve diameter is reduced, the circumferential length of the region in which the gap size is smaller than a predetermined value and which contributes to the flying of the toner is decreased with a result that the rate of flying of the toner to the photosensitive member is reduced so as to reduce the density of the print image.
The known arts also lack any consideration as to the influence of offset or eccentricity of the photosensitive member (referred to as "photosensitive drum") which is constructed in the form of a cylindrical drum. Namely, any eccentricity of the photosensitive drum causes a fluctuation in the size of the gap (referred to as developing gap) between the developing sleeve and the photosensitive drum, resulting in an unevenness of the density of the print image.
It has been a common understanding that, in non-contact development, the layer of the developer is formed as uniformly as possible and in a small thickness as possible. This is because a large thickness of the developer layer tends to cause a large fluctuation in the electric field acting on the developer layer so as to make it difficult to delicately control the electric force acting on the toner. More practically, it has been suggested that the developer layer on the sleeve optimumly has a thickness of 0.3 to 1.5 mm. To this end, it has been proposed to reduce the thickness of the developer layer by (a) reducing the gap between a thickness regulating blade and the developing sleeve or by (b) using, as the carrier, magnetic particles in the form of mixture of magnetic powders and binder resin particles so as to weaken the magnetization.
However, no proposals has been made as to the arrangement of poles of the magnet roll. For attaining a higher printing speed, it is necessary to increase the rate of fly of the toner per unit time which in turn requires that the speed of rotation of the developing sleeve to be increased. Unfortunately, however, when the gap between the thickness regulating blade and the developing sleeve is reduced, the toner tends to clog in this gap particularly when the rotation speed of the developing sleeve is increased. In consequence, lines such as those formed by brooming are formed in the toner layer on the developing sleeve. Conversely, in the system in which a carrier containing a binder is used, there is a risk for the developer to be scattered due to centrifugal force when the rotation speed is increased, because of a too small magnetic attracting force.
In the mono-component layer type method, the use of a magnetic brush as the means for applying a toner increases the size of the developing apparatus. On the other hand, the elastic-blade type developer applicator for applying toner to the developing roll tends to cause the toner and other component to stick to the developing roll, resulting in a lack of stability.
The bi-component layer type method is advantageous because it can contribute to a simplification in the construction without being accompanied by any increase in the size. This method, however, involves a risk that the carrier may fly together with the toner towards the photosensitive drum during developing or when the development is not conducted. To obviate this problem, Japanese Patent Unexamined Publication No. 60-242469 proposes a method in which an electrically insulating carrier is used to prevent the carrier from flying. The use of electrically insulating carrier, however, is disadvantageous in that the carrier may undesirably fly towards the photosensitive member by application of an electric field of a reverse polarity.